Near-vertical incidence HF radar

ABSTRACT

An HF radar system comprises a transmitting system, a receiving system, a signal processing system and a frequency management/ionospheric sounding system. The transmitting system comprises a transmitting antenna array configured to transmit a beam in a near vertical direction and a transmitting device arranged to drive the transmitting antenna array at frequencies suitable for downward refraction by the ionosphere. The receiving system comprises a receiving antenna array configured to receive returning signals from a target area returning to the receiving antenna array via refraction at the ionosphere. The signal processing system comprises a digital data processing system. The frequency management/sounding system comprises cooperating transmitting and receiving systems sending HF signals to the ionosphere and analysing the returning signals. Alternatively, the system may have a duplexed antenna array. The receiving system includes means to discriminate the returning signal produced by a helicopter, other aircraft or surface vessels.

This application is the U.S. national phase of international applicationPCT/GB01/03206, filed in English on 17 Jul. 2001 which designated theU.S. PCT/GB01/03206 claims priority to GB Application No. 0018170.1filed 26 Jul. 2000. The entire contents of these applications areincorporated herein by references.

The present invention relates to a use of HF Radar. It particularlyrelates to HF Radar installations consisting of arrays of receiving andtransmitting antennas configured to produce near-vertical incidence ofradiation paths to and from the ionosphere overhead to illuminate a landor sea area near to the antennas with horizontally polarised radiation,HF Radar being normally specified to be in the frequency range of 2-30MHz.

As depicted in FIG. 1, whereas microwave radar is generally limited toline-of-sight surveillance, HF Surfacewave Radar allows‘over-the-horizon’ surveillance to be made. This is due toelectromagnetic waves at HF having the important property of beingrefracted by the ionosphere so as to return to earth. As depicted inFIG. 2, HF Skywave Radar also allows ‘over-the-horizon’ surveillance butwith the constraint that detections cannot be made at distances lessthan some 600 km, the so-called “skip distance”.

It is the practice to transmit vertically polarised signals in HFSurfacewave Radar to achieve coupling to the conducting surface of thesea. Vertical polarisation is also used for HF Skywave radar for ease ofantenna construction.

Where the radar target is a helicopter, the Skywave form of HF Radar isknown to obtain detections due to the downward signal from theionosphere being reflected from the rotating blades (the downward signalis horizontally polarised due to the refraction mechanisms at theionosphere). The reflections from the multiple blade rotor rotationresult in a characteristic, identifiable modulation of the HF Radarsignal.

Where detections by microwave radar of targets such as helicopters,other aircraft or surface vessels are to be made at short distances fromthe radar, typically 20 km to 150 km, intervening topography such asmountainous terrain, may prohibit ‘above-the-horizon’ radar detection.

Lack of sea in the foreground could prohibit HF Surfacewave‘below-horizon’ detection, also its obligatory vertical polarisation isnot in the horizontal plane which is required for optimal reflectionfrom the rotating near horizontal rotor blades of helicopters; theminimum skip-distance criterion will prohibit detection by conventionalHF Skywave transmission. In these cases recourse could be made tosurveillance over-flights of the target area by rotary or fixed wingaircraft or by satellite borne sensors.

For operational reasons, or if the target is stealthy to microwavedetection, these detection methods may be limited in application. Inparticular, a slow-moving, low-flying helicopter would be difficult todetect when shielded by terrain or where the background produces radarclutter.

In order to produce a response from the rotor blades of a low-flyinghelicopter with horizontally polarised energy in the HF radio band, themode of transmission and reception called Near-Vertical IncidenceSkywave (NVIS) will be utilised, where horizontally polarised radiationis launched from a suitably configured HF transmitting antenna array indirections lying within an inverted cone of some 30°-apex angle. Bysuitable choices of radiated frequency within the HF band, downwardrefraction can be achieved over a significant part of the 24-hourdiurnal sun cycle (the sun's radiation causes the necessary ionisationfor producing this refraction).

The downward-travelling signal illuminates the earth's surface togetherwith targets including ships and aircraft moving over it. Back-scatteredreturns from these will travel upwards in a similar path direction whichwill allow a further refraction at the ionosphere causing the signal totravel down again to the vicinity of the transmitting site. Normally,near to the transmitting antenna array is located a horizontallypolarised upwardly-directed receiving antenna array for intercepting thereturns from the illuminated target area.

The present invention provides an arrangement whereby the disadvantagesof the prior art are overcome by obtaining returns from targets whichmay be hidden from normal sensors by mountainous terrain, steep-sidedfjords, by below-the-horizon limitations, or where the target may bestealthy (with reduced visibility) to microwave radar.

According to the present invention there is provided an HF radar systemcomprising a transmitting system, a receiving system, a signalprocessing system and a frequency management/ionospheric soundingsystem;

the transmitting system comprising a transmitting antenna arrayconfigured to transmit a beam in a near vertical direction and atransmitting device arranged to drive the transmitting antenna array atfrequencies suitable for downward refraction by the ionosphere;

the receiving system comprising a receiving antenna array configured toreceive returning signals from a target area returning to the receivingantenna array via refraction at the ionosphere;

the signal processing system comprising a digital data processingsystem;

the frequency management/sounding system comprising a cooperatingtransmitting system and receiving system sending HF signals to theionosphere and analysing the returning signals.

The receiving system includes means to discriminate a returning signalfrom a helicopter.

Alternatively, according to the present invention there is provided anHF radar system comprising a duplexed antenna array, a transmittingsystem, a receiving system, a signal processing system and a frequencymanagement/ionospheric sounding system;

the duplexed antenna array comprising a composite directionaltransmitting and receiving antenna array connected to a duplexer andconfigured to transmit a beam in a near vertical direction and toreceive returning signals from a target area returning to the compositetransmitting and receiving antenna array via refraction at theionosphere;

the transmitting system comprising a transmitting device being connectedvia the duplexer to and arranged to drive the composite directionaltransmitting and receiving antenna array at frequencies suitable fordownward refraction by the ionosphere;

the receiving system being connected to the duplexer and beingconfigured to receive returning signals from the target area returningto the receiving antenna array via refraction at the ionosphere;

the signal processing system comprising a digital data processingsystem;

the frequency management/sounding system comprising a cooperatingtransmitting system and receiving system sending HF signals to theionosphere and analysing the returning signals.

The present invention will now be described, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 shows the typical surveillance range of HF Surfacewave Radar;

FIG. 2 shows the typical surveillance range of HF Skywave Radar;

FIG. 3 shows a block diagram of an example of an NVIS Radar according tothe present invention; and

FIG. 4 shows a block diagram of an example of an NVIS Radar according tothe present invention including an antenna duplexer.

As shown in FIG. 3, the radar system comprises:

a transmitting system, a receiving system, a signal processing system,and a frequency management/ionospheric sounding system, which frequencymanagement/ionospheric sounding system can be part of the present radarsystem with which it may be used on a time-shared basis.

The transmitting system comprises:

a directional antenna array 11, configured to transmit a beam in nearvertical directions for illumination of a section of the overheadionosphere 12.

a transmitting device 13 which will drive the above mentioned array 11at frequencies which are suitable for almost total downward refractionfrom the ionosphere 12 and carry modulation appropriate todiscrimination of targets at a target area 22, from background radionoise and land, sea and ionospheric clutter at a target area 22;

The receiving system comprises:

a directional receiving antenna array 21 configured and directed toreceive signals scattered from the target area 22 and returning to thelocation of the receiving antenna array 21 by refraction at theionosphere 12 overhead;

a receiving device 23 with conversion means for conversion of thesignals received by the receiving antenna 21 into a digital streamcarrying electrical descriptors of targets detected in the groundillumination foot-print, also of clutter and noise signals.

The signal processing system comprises:

a digital data processing system 31, which allows discrimination of thecharacteristic returns from the target, and in particular discriminationof returns modulated by the motion of the multiple rotor blades of ahelicopter, from the returns due to land, to the sea, and to the varyingionosphere 12. The processed signals are then further converted forpresentation on a radar display 32.

The frequency management/sounding system comprises:

a transmitting system 41 and a receiving system 42 which operatetogether, to send HF signals to the ionosphere 13, and analyse thosesignals upon return to earth, to provide information on the height ofthe refracting ionosphere 13 layer above the earth, the areadistribution of the layer and the optimum frequencies for this 2-waysignal path.

The target area 13 will be typically an annular area having 20 Km innerradius and 150 Km outer radius.

As shown in FIG. 4, rather than having separate transmitting andreceiving antenna arrays 11, 21, the transmitting device 13 and thereceiving device 23 may be connected to an antenna duplexer 51 having acommon directional transmitting and receiving antenna 52.

What is claimed is:
 1. An HF radar system comprising: a transmittingsystem comprising a directional transmitting antenna array configured totransmit a beam in an upward direction and a transmitting devicearranged to drive the transmitting antenna array at frequencies suitablefor downward refraction by the ionosphere; a receiving system comprisinga directional receiving antenna array configured to receive returningsignals reflected from a target area returning to the receiving antennaarray via refraction at the ionosphere; a signal processing systemcomprising a digital data processing system; and a frequency managementand ionospheric sounding system, said sounding system comprising acooperating transmitting system and receiving system sending HF signalsto the ionosphere and analysing the returning signals.
 2. An HF radarsystem as claimed in claim 1, wherein the transmitting device providesmodulation to enable the discrimination of targets from background noiseand land, sea and ionospheric clutter.
 3. An HF radar system as claimedin claim 1, wherein the receiving system further comprises conversionmeans for conversion of returning signals received by the receivingantenna into a digital stream carrying electrical descriptors of targetsdetected.
 4. An HF radar system as claimed in claim 1, wherein thesignal processing system comprises a discriminator to providediscrimination of characteristic returns from a target.
 5. An HF radarsystem as claimed in claim 4, wherein the discriminator providesdiscrimination of the characteristic return from a helicopter.
 6. An HFradar system as claimed in claim 1, further comprising means fordisplaying the processed signals.
 7. An HF radar system as claimed inclaim 1, wherein the frequency management and sounding system providesinformation regarding the height of the refracting ionosphere layerabove the earth, the area distribution of the layer and the optimumfrequencies for the 2-way signal path and controls the transmissionfrequency at a current optimum frequency.
 8. An HF radar system asclaimed in claim 1, wherein the frequency management and sounding systemcomprises the transmitting system, receiving system and signalprocessing system used on a time-shared basis.
 9. An HF radar system asclaimed in claim 1, wherein the target area is an annular area having aninner radius of 20 Km approximately and an outer radius of 150 Kmapproximately.
 10. An HF radar system comprising: a duplexed antennaarray, said duplexed antenna array comprising a composite directionaltransmitting and receiving antenna array connected to a duplexer andconfigured to transmit a beam in an upward direction and to receivereturning signals reflected from a target area returning to thecomposite transmitting and receiving antenna array via refraction at theionosphere; a transmitting system comprising a transmitting device beingconnected via the duplexer to and arranged to drive the compositedirectional transmitting and receiving antenna array at frequenciessuitable for downward refraction by the ionosphere; a receiving systemconnected to the duplexer and being configured to receive returningsignals from the target area returning to the receiving antenna arrayvia refraction at the ionosphere; a signal processing system, the signalprocessing system comprising a digital data processing system; and afrequency management and ionospheric sounding system comprising acooperating transmitting system and receiving system sending HF signalsto the ionosphere and analysing the returning signals.